1. Field of the Invention
The invention is a method of sawing bar-shaped workpieces into slices by means of an annular saw. The inner margin of the rotating annular saw blade is set with cutting grains. The rotating saw blade is clamped into a clamping system at its outer periphery and is guided radially through the workpiece held in the cutting position by means of a workpiece holding device. The deviation of the actual position of the saw blade from a nominal position is detected and the relative position of the saw blade and workpiece is adjusted during the sawing operation. The invention also comprises annular saws for performing the method.
To precisely divide bar-shaped workpieces, for example, made of glass, quartz, ceramic or oxidic material, such as ruby or gallium gadolinium garnet or compound semiconductor materials such as gallium arsenide or indium phosphide and, in particular, elementary semiconductor materials as, silicon or germanium into slices, annular saws are used in most cases. Annular saws can provide slices which can meet the often stringent requirements imposed on the geometric quality of the products. Here, circular thin-sheet disks which, concentric to the outer periphery, have a central circular bore, the inner margin of the circular bore serving as a saw blade. The inner peripheral area of the disk is provided with a cutting edge which generally comprises grains of hard material embedded in a bond matrix.
For example, a cutting edge having a drop-shaped cross-section comprising diamond or boron nitride grains embedded in a metal matrix, preferably a nickel matrix, have proved successful for sawing bars of semiconductor material. Bars of silicon have been sawed into thin slices of about 0.1 to 1 mm. thickness for use, for example, in the manufacture of electronic or power components.
The saw blade, at its outer periphery, is clamped into a clamping system. Useful clamping systems are disclosed in accordance with the embodiments described in German Patent No. 3,442,730, U.S. Pat. Nos. 3,039,235, 3,175,548 and German Patent No. 2,841,653. The saw blade is arranged as coaxially and as rotationally symmetric as possible on the sawing machine rotor which, as a rule, is opened out in a cup-like fashion. The workpiece to be sawed, e.g., a silicon bar, is generally fixed in the workpiece holding device by means of an adapter device and is moved via an infeed mechanism into the intended cutting position and held there.
During the sawing operation, the rotating saw blade and the workpiece are subjected to a relative movement, by means of which the cutting edge works radially through the workpiece until finally the desired slice is cut off. In the case of the known annular saws, different variants are used for this purpose. In one embodiment, the workpiece is moved toward the rotating saw blade, and the rotating saw blade remains in a fixed position. In another embodiment, the saw blade is moved through the workpiece mounted in a fixed position; in this embodiment, the workpiece can be located in either a horizontal or a vertical position with the saw blade being correspondingly vertically or horizontally mounted.
In each of these variants, however, apart from the radially directed main cutting force, additional forces directed axially, i.e., perpendicularly to the saw blade plane, generally act on the saw blade during the cutting-off operation. These forces affect its motion in the workpiece and can lead to deviations from the desired cutting line. For example, irregularities in the cutting edge such as, for example, non-uniform wear, varying density in the set cutting grains, non-uniform cutting grain geometry or varying accumulation of removed material on the cutting edge can be possible causes of these forces. In addition, irregular pressing conditions in the sawing gap or displacement of the workpiece or of the sawing machine rotor can also cause these axial forces.
As a result of the deflection of the saw blade, deviations from the shape actually desired appear in the product. If, for example, the saw blade during two successive sawing operations is, in each case, deflected from the ideal position to the same extent, the surfaces of a slice which are produced by these two cuts, are not ideally flat, but are curved in parallel. The slice has no thickness variation but it does have a wrap or a curvature. If the deflections are in opposite directions, slices having a thickness variation result. For characterizing the slices with regard to such geometric errors, standardized test methods have been developed. The method according to the ASTM Standard F 657-80 is useful for determining the values for the warp and/or the bow of a slice. Satisfactory slices may deviate from a predetermined nominal geometry only within narrow tolerance limits. Semiconductor slices for use in the manufacture of electronic especially highly integrated components and, in particular, in photolithographic processes, are subject to particularly strict requirements of the geometric precision.
2. RELATED ART
In the method according to DE-A-3,640,645 (corresponding U.S. Pat. No. 4,844,047), the progress of the cut is monitored by means of measuring devices during the sawing operation and the deviation of the saw blade is counteracted by application of a fluid to at least one side face of the saw blade before entry into the sawing gap. Here, the pressure conditions in the sawing gap change and a force component opposed to the deflecting force is produced so that the nominal cutting line can be maintained more accurately. However, this method is complicated in terms of apparatus and regulating means, especially as these regulating forces produced in a fluid-dynamic manner are, from experience, relatively weak. In addition, the regulating forces only become effective when a certain penetration depth of the saw blade in the material to be sawed is reached.
The object of the invention is a method which is less complicated in terms of regulating means and apparatus, and enables stringent requirements imposed on the geometric quality of the slices to be maintained, even in the case of large slice diameters. Furthermore, it is an object to provide a method which can be used with existing annular saws without expensive conversion. The method permits not only the manufacture of flat slices, but also, such slices having a certain bending (bow) on one or both surfaces. Furthermore, the invention is the annular saws suitable for performing the method.